tests: more interface tests; checker: handle zero selectorexpr type error

vlib/v/checker/checker.v

@@ -481,6 +481,10 @@ fn (mut c Checker) fail_if_immutable(expr ast.Expr) {
 		}
 		ast.SelectorExpr {
 			// retrieve table.Field
+			if it.expr_type == 0 {
+				c.error('0 type in SelectorExpr', expr.position())
+				return
+			}
 			typ_sym := c.table.get_type_symbol(it.expr_type)
 			match typ_sym.kind {
 				.struct_ {

vlib/v/tests/interface_test.v

@@ -56,22 +56,9 @@ fn test_perform_speak() {
 
 fn perform_speakers(speakers []Speaker) {}
 
-/*
-interface Speak2er {
-	name ()string
-	speak()
-	}
-
-
-struct Foo {
-	speaker  Speaker
-	speakers []Speaker
-}
-
-
-
 interface Register {
-	register()}
+	register()
+}
 
 struct RegTest {
 	a int
@@ -82,12 +69,19 @@ fn (f RegTest) register() {}
 fn handle_reg(r Register) {}
 
 fn test_register() {
-	if true {
-		// QTODO
-		return
-	}
 	f := RegTest{}
 	f.register()
 	handle_reg(f)
 }
-*/
+
+interface Speaker2 {
+	name() string
+	speak()
+}
+
+
+struct Foo {
+	//speaker  Speaker
+	//speakers []Speaker
+}
+